Appearance. The SA-X is an exact copy of Samus Aran, or at least her suit. It's unknown what the SA-X really looks like underneath the armor, but it does have a rather creepy, expressionless, zombie-looking face with missing eye pupils. Its gender is also a mystery.

^ Adam: You know that the X reproduce by asexual division. Because of this mechanism, there are now no fewer than 10 SA-X aboard the station. Nintendo R&D1. Metroid Fusion.

"X seem to have originated on SR388, which is also home to their sole predator, the Metroids. Since Samus Aran exterminated the Metroids on SR388, the X have overrun and infected most of the life-forms on the planet." "X infected other creatures and grow in the nervous systems of these host organisms.

Remember, the X and Metroids are "natural" enemies. In Fusion, the SA-X was willing to die in order to destroy the Metroid lab and an X core willing to blow up BSL along itself. Later, a SA-X "saved" Samus from the Omega Metroid which in reality was just a self-preservation method.